Steel material suitable for use in sour environment

ABSTRACT

A steel material according to the present disclosure has a chemical composition consisting of, in mass %: C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, and the balance being Fe and impurities, and satisfying Formula (1) described in the specification. A grain diameter of a prior-austenite grain is 11.0 μm or less, and an average area of precipitate which is precipitated in a prior-austenite grain boundary is 10.0×10 −3  μm 2  or less. A yield strength is 758 to 862 MPa.

TECHNICAL FIELD

The present disclosure relates to a steel material, and moreparticularly to a steel material suitable for use in a sour environment.

BACKGROUND ART

Due to the deepening of oil wells and gas wells (hereunder, oil wellsand gas wells are collectively referred to as “oil wells”), there is ademand to enhance the strength of oil-well steel materials representedby oil-well steel pipes. Specifically, 80 ksi grade (yield strength is80 to less than 95 ksi, that is, 552 to less than 655 MPa) and 95 ksigrade (yield strength is 95 to less than 110 ksi, that is, 655 to lessthan 758 MPa) oil-well steel pipes are being widely utilized, andrecently requests are also starting to be made for 110 ksi grade (yieldstrength is 110 to 125 ksi, that is, 758 to 862 MPa) oil-well steelpipes.

Most deep wells are in a sour environment containing corrosive hydrogensulfide. In the present description, a sour environment means anenvironment which contains hydrogen sulfide, and which is acidified.Note that a sour environment may contain carbon dioxide. Oil-well steelpipes used in such a sour environment are required to have not only highstrength but also sulfide stress cracking resistance (hereinafter,referred to as “SSC resistance”).

A technique of increasing SSC resistance of a steel material, such as anoil-well steel pipe, is disclosed in Japanese Patent ApplicationPublication No. 62-253720 (Patent Literature 1), Japanese PatentApplication Publication No. 59-232220 (Patent Literature 2), JapanesePatent Application Publication No. 06-322478 (Patent Literature 3),Japanese Patent Application Publication No. 08-311551 (Patent Literature4), Japanese Patent Application Publication No. 2000-256783 (PatentLiterature 5). Japanese Patent Application Publication No. 2000-297344(Patent Literature 6), Japanese Patent Application Publication No.2005-350754 (Patent Literature 7), National Publication of InternationalPatent Application No. 2012-519238 (Patent Literature 8), and JapanesePatent Application Publication No. 2012-26030 (Patent Literature 9).

Patent Literature 1 proposes a method for increasing SSC resistance ofsteel for oil well by reducing impurities, such as Mn and P. PatentLiterature 2 proposes a method for increasing SSC resistance of steel byperforming quenching two times to make grain fine.

Patent Literature 3 proposes a method for increasing SSC resistance of asteel material having 125 ksi grade by making the micro-structure ofsteel fine by induction heat treatment. Patent Literature 4 proposes amethod for increasing SSC resistance of a steel pipe having 110 to 140ksi grade by increasing the hardenability of steel by utilizing directquenching process and also by increasing a tempering temperature.

Patent Literature 5 and Patent Literature 6 propose a method forincreasing SSC resistance of steel for low alloy oil country tubulargoods having 110 to 140 ksi grade by controlling the morphology ofcarbide. Patent Literature 7 proposes a method for increasing SSCresistance of a steel material having 125 ksi grade or more bycontrolling dislocation density and a hydrogen diffusion coefficient topredetermined values. Patent Literature 8 proposes a method forincreasing SSC resistance of steel having 125 ksi grade by performingquenching a plurality of times on low alloy steel which contains C of0.3 to 0.5%. Patent Literature 9 proposes a method for controlling themorphology and the number of carbide by adopting a tempering process oftwo-stage heat treatment. More specifically, in Patent Literature 9, anumber density of large-sized M₃C or M₂C is suppressed to increase SSCresistance of steel having 125 ksi grade.

CITATION LIST Patent Literature [Patent Literature 1] Japanese PatentApplication Publication No. 62-253720 [Patent Literature 2] JapanesePatent Application Publication No. 59-232220 [Patent Literature 3]Japanese Patent Application Publication No. 06-322478 [Patent Literature4] Japanese Patent Application Publication No. 08-311551 [PatentLiterature 5] Japanese Patent Application Publication No. 2000-256783[Patent Literature 6] Japanese Patent Application Publication No.2000-297344 [Patent Literature 7] Japanese Patent ApplicationPublication No. 2005-350754 [Patent Literature 8] National Publicationof International Patent Application No. 2012-519238 [Patent Literature9] Japanese Patent Application Publication No. 2012-26030 SUMMARY OFINVENTION Technical Problem

However, a steel material (an oil-well steel pipe, for example)exhibiting yield strength of 110 ksi (758 to 862 MPa) and excellent SSCresistance may be obtained by a technique other than techniquesdisclosed in the aforementioned Patent Literatures 1 to 9.

It is an objective of the present disclosure to provide a steel materialwhich has yield strength of 758 to 862 MPa (110 ksi grade), and also hasexcellent SSC resistance in a sour environment.

Solution to Problem

A steel material according to the present disclosure has a chemicalcomposition consisting of, in mass %: C: 0.20 to 0.45%, Si: 0.05 to1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al:0.005 to 0.100%, Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00%, Ti:0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg:0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, and W: 0 to 0.50%, with thebalance being Fe and impurities, and satisfying Formula (1). In themicrostructure of the steel material, a grain diameter of aprior-austenite grain is 11.0 μm or less. An average area of precipitatewhich is precipitated in a prior-austenite grain boundary is 10.0×10⁻³μm² or less in the steel material. A yield strength of the steelmaterial is 758 to 862 MPa.

$\begin{matrix}{{{2.7 \times C} + {0.4 \times {Si}} + {Mn} + {0.45 \times {Ni}} + {0.45 \times {Cu}} + {0.8 \times {Cr}} + {2 \times {Mo}}} \geq 3.90} & (1)\end{matrix}$

where, content (mass %) of a corresponding element is substituted for anelement symbol in Formula (1), and when the corresponding element is notcontained, “0” is substituted for the element symbol.

Advantageous Effects of Invention

The steel material according to the present disclosure has yieldstrength of 758 to 862 MPa (110 ksi grade), and also has excellent SSCresistance in a sour environment.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a view showing the relationship between Mo content and prior γgrain diameter.

DESCRIPTION OF EMBODIMENT

The present inventors have conducted investigations and studiesregarding a method for obtaining excellent SSC resistance of a steelmaterial which is expected to be used in a sour environment while yieldstrength of 758 to 862 MPa (110 ksi grade) is maintained. As a result,the following findings are obtained.

Increasing dislocation density in the steel material increases yieldstrength YS of the steel material. Meanwhile, there is a possibilitythat dislocations in the steel material occlude hydrogen. Therefore,when dislocation density in the steel material is increased, the amountof hydrogen occluded by the steel material may be increased. Whenhydrogen concentration in the steel material is increased as a result ofan increase in dislocation density, high strength may be obtained, butSSC resistance of the steel material is reduced. Accordingly, to achieveboth yield strength of 110 ksi grade and excellent SSC resistance, it isnot preferable to increase strength by making use of dislocationdensity.

In view of the above, the present inventors considered that when yieldstrength of a steel material is increased using a method different froman increase in dislocation density of the steel material, excellent SSCresistance may be obtained even if yield strength of the steel materialis increased to 110 ksi grade.

Specifically, the present inventors have considered that a steelmaterial having the chemical composition including, in mass %: C: 0.20to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S:0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Ti: 0.002 to0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N:0.0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, and W: 0 to 0.50%, may achieveboth yield strength of 110 ksi grade and SSC resistance.

The present inventors further considered that when Mo is contained inaddition to the aforementioned chemical composition, alloy carbide isformed and hence, yield strength may be increased without increasingdislocation density excessively. Accordingly, the present inventorsproduced various steel materials where Mo is added to the aforementionedchemical composition, and investigated characteristics of the steelmaterials. As a result, the present inventors have newly found that, inthe steel material having the aforementioned chemical composition, Mocontent and the grain diameter of prior-austenite grain (hereinafter,also referred to as “prior γ grain”) have dependencies.

Specifically, the relationship between Mo content and a prior γ graindiameter will be described with reference to a drawing. FIG. 1 is a viewshowing the relationship between Mo content and a prior γ graindiameter. FIG. 1 is formed using Mo contents (mass %) and prior γ graindiameters (μm) acquired by microstructure observation described laterwith respect to steel materials which have the chemical compositionother than Mo content satisfying the range of the aforementionedchemical composition, and which are produced by a preferred productionmethod described later in an example which will be described later. Inthe present description, “prior γ grain diameter” means the graindiameter of a prior γ grain obtained by a method conforming to acomparison method defined in ASTM E112-10.

Referring to FIG. 1, when Mo content increases, the prior γ graindiameter is dramatically reduced. It became apparent that, in the steelmaterial having the aforementioned chemical composition, when the Mocontent becomes more than 1.00%, notable advantageous effect of reducinga prior γ grain diameter to 11.0 μm or less is obtained. Further, when aprior γ grain is fine, the steel material can increase both the yieldstrength and the SSC resistance. Accordingly, the chemical compositionof the steel material according to the present embodiment contains Mo ofmore than 1.00 to 2.00% in addition to the aforementioned chemicalcomposition. In this case, the prior γ grain diameter in the steelmaterial becomes 11.0 μm or less.

The present inventors consider the reason as follows. In the case wherethe steel material having the aforementioned chemical compositioncontains Mo of more than 1.00 to 2.00%, there is a possibility that Modissolved in the steel material segregates in austenite grain boundariesduring heating in a quenching process. Accordingly, dissolved Mosegregated in austenite grain boundaries suppresses the movement ofgrain boundaries. As a result, austenite grain is prevented from beingeasily coarsened during heating in a quenching process and hence, it isconsidered that prior γ grain on which tempering is performed is madefine.

Meanwhile, to cause the steel material having the aforementionedchemical composition to obtain yield strength of 110 ksi grade, higherhardenability of the steel material is preferable. In the presentdescription, F1 is defined as2.7×C+0.4×Si+Mn+0.45×Ni+0.45×Cu+0.8×Cr+2×Mo. F1 is the index of thehardenability of the steel material. When F1 is too low, there may be acase where sufficient hardenability of the steel material cannot beobtained so that yield strength of 110 ksi grade cannot be obtained.Accordingly, the steel material according to the present embodiment hasthe aforementioned chemical composition, and further has F1 of 3.90 ormore.

Therefore, the steel material according to the present embodiment has achemical composition consisting of, in mass %: C: 0.20 to 0.45%, Si:0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less,Al: 0.005 to 0.100%, Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00%, Ti:0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg:0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, W: 0 to 0.50%, and the balancebeing Fe and impurities, and the aforementioned F1 is 3.90 or more.Further, in the microstructure of the steel material according to thepresent embodiment, a prior γ grain diameter is 11.0 μm or less.

However, in the steel material having the aforementioned chemicalcomposition and the prior γ grain diameter of 11.0 μm or less, when anattempt is made to obtain yield strength of 110 ksi grade, there may bea case where a large amount of coarse carbide is precipitated in thesteel material. As a result of further investigation performed by thepresent inventors, it has been found that, when a large amount of coarsecarbide is precipitated in the steel material having the aforementionedchemical composition, the steel material may not obtain excellent SSCresistance in a sour environment.

Accordingly, the present inventors have discussed in more detail withrespect to carbide which reduces SSC resistance in the steel materialhaving the aforementioned chemical composition. As a result, thefollowing findings have been acquired. Coarse carbide is liable to formstress concentrators, and promotes propagation of cracks caused by SSC.Therefore, it has been considered that reducing coarse carbide increasesSSC resistance of a steel material.

However, as the result of detailed discussion performed by the presentinventors, the present inventors have found that, of the coarse carbide,particularly, coarse carbide which is precipitated in prior γ grainboundaries may cause a reduction in SSC resistance of a steel material.That is, the present inventors have found that SSC resistance of a steelmaterial can be increased not by simply reducing coarse carbide but byreducing coarse carbide which is precipitated in the prior γ grainboundaries.

In the steel material according to the present embodiment having theaforementioned chemical composition, most of the precipitates which areprecipitated in the prior γ grain boundary are carbide. Accordingly,reducing coarse precipitates which are precipitated in the prior γ grainboundaries can reduce coarse carbide which is precipitated in the priorγ grain boundaries.

Accordingly, the steel material according to the present embodiment hasthe aforementioned chemical composition and a prior γ grain diameter of11.0 μm or less and, further, reduces coarse carbide which isprecipitated in the prior γ grain boundaries. Specifically, the steelmaterial according to the present embodiment has the aforementionedchemical composition and the prior γ grain diameter of 11.0 μm or less.Further, the average area of the precipitates in the prior γ grainboundary is 10.0×10⁻³ μm² or less. As a result, the steel materialaccording to the present embodiment can achieve both yield strength of758 to 862 MPa (110 ksi grade) and excellent SSC resistance in a sourenvironment.

The steel material according to the present embodiment completed basedon the aforementioned findings has the chemical composition consistingof, by mass %, C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%,P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.60 to1.50%, Mo: more than 1.00 to 2.00%, Ti: 0.002 to 0.020%, V: 0.05 to0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O:less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%,rare earth metal: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to0.50%, and W: 0 to 0.50%, with the balance being Fe and impurities, andsatisfying Formula (1). In the microstructure of the steel material, thegrain diameter of a prior-austenite grain is 11.0 μm or less. In thesteel material, the average area of precipitates which are precipitatedin the prior-austenite grain boundary is 10.0×10⁻³ μm² or less. A yieldstrength of the steel material is 758 to 862 MPa.

$\begin{matrix}{{{2.7 \times C} + {0.4 \times {Si}} + {Mn} + {0.45 \times {Ni}} + {0.45 \times {Cu}} + {0.8 \times {Cr}} + {2 \times {Mo}}} \geq 3.90} & (1)\end{matrix}$

where, content (mass %) of a corresponding element is substituted foreach symbol of an element in Formula (1), and if a corresponding elementis not contained, “0” is substituted for the element symbol of therelevant element.

In the present description, the steel material is not particularlylimited. However, the steel material may be a steel pipe or a steelplate, for example.

The steel material according to the present embodiment exhibits yieldstrength of 758 to 862 MPa (110 ksi grade) and excellent SSC resistance.

The aforementioned chemical composition may contain one or more types ofelement selected from the group consisting of Ca: 0.0001 to 0.0100%, Mg:0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and rare earth metal: 0.0001to 0.0100%.

The aforementioned chemical composition may contain one or more types ofelement selected from the group consisting of Cu: 0.02 to 0.50%, and Ni:0.02 to 0.50%.

The aforementioned chemical composition may contain one or more types ofelement selected from the group consisting of Co: 0.02 to 0.50%, and W:0.02 to 0.50%.

The aforementioned steel material may be an oil-well steel pipe.

In the present description, the oil-well steel pipe may be a steel pipethat is used for a line pipe or may be a steel pipe used for oil countrytubular goods (OCTG). The shape of the oil-well steel pipe is notlimited, and for example, the oil-well steel pipe may be a seamlesssteel pipe or may be a welded steel pipe. The oil country tubular goodsare, for example, steel pipes that are used for use in casing or tubing.

The aforementioned steel material may be a seamless steel pipe. When thesteel material according to the present embodiment is a seamless steelpipe, even if a wall thickness is 15 mm or more, the steel material hasyield strength of 758 to 862 MPa (110 ksi grade), and also has morestable SSC resistance in a sour environment.

The aforementioned excellent SSC resistance can be evaluatedspecifically by a method in accordance with “Method A” specified in NACETM0177-2005 and a four-point bending test. In the method in accordancewith “Method A” specified in NACE TM0177-2005, a mixed aqueous solutioncontaining 5.0 mass % of sodium chloride and 0.5 mass % of acetic acid(NACE solution A) at 4° C. is employed as a test bath. A stressequivalent to 90% of the actual yield stress is applied to the testspecimen that is taken from the steel material according to the presentembodiment, and the test specimen is immersed into the test bath. Thetest bath is degassed, thereafter, H₂S gas at 1 atm is blown into thetest bath to cause saturation of the H₂S gas. The test bath wheresaturation of the H₂S gas is caused is held for 720 hours at 4° C.

Meanwhile, in the four-point bending test, stress is applied to a testspecimen taken from the steel material by four-point bending inaccordance with ASTM G39-99 (2011) such that stress applied to the testspecimen is set to 90% of actual yield stress of the steel material. 5.0mass % sodium chloride aqueous solution at 24° C. is employed as a testbath. The test specimen to which stress is applied is immersed into thetest bath in the autoclave. The test bath is degassed, thereafter, H₂Sgas at 20 atm is pressure-sealed into the autoclave. After the autoclaveis sealed, the test bath is stirred for 720 hours at 24° C.

In the steel material according to the present embodiment, cracking isnot confirmed after 720 hours elapses in both the aforementioned methodin accordance with “Method A” and the four-point bending test.

Hereinafter, a steel material according to the present embodiment willbe described in detail. Unless otherwise specified, “%” in relation toan element means mass %.

[Chemical Composition]

The chemical composition of the steel material according to the presentembodiment contains the following elements.

C: 0.20 to 0.45%

Carbon (C) increases the hardenability of the steel material, thusincreasing the yield strength of the steel material. Further, C promotesspheroidization of carbides during tempering in a production process,thus the SSC resistance of the steel material is increased. When thecarbides are dispersed, the yield strength of the steel material isfurther increased. When the C content is too low, these advantageouseffects cannot be obtained. On the other hand, when the C content is toohigh, toughness of a steel material is reduced so that quenching cracksare liable to occur. Accordingly, the C content is within the range of0.20 to 0.45%. A preferable lower limit of the C content is 0.21%, morepreferably is 0.22%, and further preferably is 0.25%. A preferable upperlimit of the C content is 0.40%, more preferably is 0.38%, and furtherpreferably is 0.35%.

Si: 0.05 to 1.00%

Silicon (Si) deoxidizes the steel. When the Si content is too low, thisadvantageous effect cannot be obtained. On the other hand, when the Sicontent is too high, the SSC resistance of a steel material is reduced.Accordingly, the Si content is within a range of 0.05 to 1.00%. Apreferable lower limit of the Si content is 0.10%, and more preferablyis 0.15%. A preferable upper limit of the Si content is 0.85%, morepreferably is 0.70%, and further preferably is 0.60%.

Mn: 0.01 to 1.00%

Manganese (Mn) deoxidizes the steel. Mn also enhances the hardenabilityof a steel material, thus the yield strength of the steel material isincreased. When the Mn content is too low, these advantageous effectscannot be obtained. On the other hand, when the Mn content is too high,Mn segregates in grain boundaries together with impurities, such as Pand S. In this case, the SSC resistance of the steel material isreduced. Accordingly, the Mn content is within a range of 0.01 to 1.00%.A preferable lower limit of the Mn content is 0.02%, more preferably is0.03%, and further preferably is 0.10%. A preferable upper limit of theMn content is 0.80%, more preferably is 0.70%, further preferably is0.65%, further preferably is less than 0.60%, and further preferably is0.55%.

P: 0.030% or Less

Phosphorus (P) is an impurity. That is, the P content is more than 0%. Psegregates at the grain boundaries, and reduces the SSC resistance of asteel material. Accordingly, the P content is 0.030% or less. Apreferable upper limit of the P content is 0.025%, and more preferablyis 0.020%. Preferably, the P content is as low as possible. However,when the P content is excessively reduced, the production cost increasessignificantly. Accordingly, in consideration of industrial production, apreferable lower limit of the P content is 0.0001%, more preferably is0.0003%, further preferably is 0.001%, and further preferably is 0.002%.

S: 0.0050% or Less

Sulfur (S) is an impurity. That is, the S content is more than 0%. Ssegregates at the grain boundaries, and reduces the SSC resistance of asteel material. Accordingly, the S content is 0.0050% or less. Apreferable upper limit of the S content is 0.0040%, more preferably is0.0030%, and further preferably is 0.0020%. Preferably, the S content isas low as possible. However, when the S content is excessively reduced,the production cost increases significantly. Accordingly, inconsideration of industrial production, a preferable lower limit of theP content is 0.0001%, and more preferably is 0.0003%.

Al: 0.005 to 0.100%

Aluminum (Al) deoxidizes the steel. When the Al content is too low, thisadvantageous effect cannot be obtained, and the SSC resistance of asteel material is reduced. On the other hand, when the Al content is toohigh, coarse oxide-based inclusions are formed, and the SSC resistanceof the steel material is reduced. Accordingly, the Al content is withina range of 0.005 to 0.100%. A preferable lower limit of the Al contentis 0.015%, and more preferably is 0.020%. A preferable upper limit ofthe Al content is 0.080%, and more preferably is 0.060%. In the presentdescription, the “Al” content means content of “acid-soluble Al”, thatis, the content of “sol. Al”.

Cr: 0.60 to 1.50%

Chromium (Cr) increases the hardenability of the steel material, andincreasing the yield strength of the steel material. Further, Crincreases temper softening resistance, and enabling high temperaturetempering. As a result, the SSC resistance of the steel material isincreased. When the Cr content is too low, these advantageous effectscannot be obtained. On the other hand, when the Cr content is too high,coarse carbides are formed in prior γ grain boundaries in the steelmaterial. In this case, the SSC resistance of the steel material isreduced. Accordingly, the Cr content is within a range of 0.60 to 1.50%.A preferable lower limit of the Cr content is 0.62%, more preferably is0.64%, further preferably is 0.65%, further preferably is 0.67%, andfurther preferably is 0.70%. A preferable upper limit of the Cr contentis 1.40%, more preferably is 1.30%, further preferably is 1.20%, furtherpreferably is 1.10%, further preferably is less than 1.00%, and furtherpreferably is 0.95%.

Mo: More than 1.00 to 2.00%

Molybdenum (Mo) increases the hardenability of the steel material, andincreasing the yield strength of the steel material. Further, Mo isdissolved in the steel material, and a part of the dissolved Mosegregates in austenite grain boundaries during heating in a quenchingprocess. As a result, the prior γ grain diameter in the steel materialon which tempering is performed is reduced by a pinning effect. In thiscase, the SSC resistance of the steel material is increased. When the Mocontent is too low, these advantageous effects cannot be obtained. Onthe other hand, when the Mo content is too high, coarse carbides areformed in prior γ grain boundaries in the steel material. In this case,the SSC resistance of the steel material is reduced. Accordingly, the Mocontent is within a range of more than 1.00 to 2.00%. A preferable lowerlimit of the Mo content is 1.01%, more preferably is 1.05%, furtherpreferably is 1.10%, further preferably is 1.15%, and further preferablyis 1.20%. A preferable upper limit of the Mo content is 1.90%, morepreferably is 1.80%, further preferably is 1.75%, further preferably is1.70%, and further preferably is 1.65%.

In the chemical composition of the steel material according to thepresent embodiment, it is preferable that the Mo content is less than2.00 times as large as the Cr content. When the Mo content is too highwith respect to the Cr content, there may be a case where prior γ grainof the steel material is coarsened. The reason for such a phenomenon isnot yet clear. However, in the steel material having the chemicalcomposition of the present embodiment, when the Mo content is less than2.00 times as large as the Cr content, the prior γ grain diameter in thesteel material can be stably set to 11.0 μm or less. Accordingly, in thechemical composition of the steel material according to the presentembodiment, it is preferable that the Mo content is less than 2.00 timesas large as the Cr content.

A preferable upper limit of the ratio of the Mo content to the Crcontent (Mo/Cr ratio) is 1.98, more preferably is 1.95, and furtherpreferably is 1.90. A preferable lower limit of the Mo/Cr ratio is notparticularly limited. However, in the chemical composition of the steelmaterial according to the present embodiment, the lower limit of theMo/Cr ratio is substantially 0.67 or more.

Ti: 0.002 to 0.020%

Titanium (Ti) forms nitride, and refines the microstructure of the steelmaterial by the pinning effect. As a result, the SSC resistance of thesteel material is increased. When the Ti content is too low, thisadvantageous effect cannot be obtained. On the other hand, when the Ticontent is too high, a large amount of Ti nitride is formed. As aresult, the SSC resistance of the steel material is reduced.Accordingly, the Ti content is within a range of 0.002 to 0.020%. Apreferable lower limit of the Ti content is 0.003%, and more preferablyis 0.004%. A preferable upper limit of the Ti content is 0.018%, andmore preferably is 0.015%.

V: 0.05 to 0.30%

Vanadium (V) combines with C and/or N to form carbides, nitrides, orcarbo-nitrides (hereinafter, referred to as “carbo-nitrides and thelike”). Carbo-nitrides and the like refines the microstructure of thesteel material by the pinning effect. As a result, the SSC resistance ofthe steel material is increased. V also combines with C to form finecarbides. As a result, the yield strength of the steel material isincreased. When the V content is too low, these advantageous effectscannot be obtained. On the other hand, when the V content is too high,carbo-nitrides and the like is excessively formed, and the SSCresistance of the steel material is reduced. Accordingly, the V contentis within a range of 0.05 to 0.30%. A preferable lower limit of the Vcontent is more than 0.05%, more preferably is 0.06%, further preferablyis 0.07%, and further preferably is 0.09%. A preferable upper limit ofthe V content is 0.25%, more preferably is 0.20%, and further preferablyis 0.15%.

Nb: 0.005 to 0.100%

Niobium (Nb) combines with C and/or N to form carbo-nitrides and thelike. Carbo-nitrides and the like refines the microstructure of thesteel material by the pinning effect. As a result, the SSC resistance ofthe steel material is increased. Nb also combines with C to form finecarbides. As a result, the yield strength of the steel material isincreased. When the Nb content is too low, these advantageous effectscannot be obtained. On the other hand, when the Nb content is too high,carbo-nitrides and the like is excessively formed, and the SSCresistance of the steel material is reduced. Accordingly, the Nb contentis within a range of 0.005 to 0.100%. A preferable lower limit of the Nbcontent is 0.007%, more preferably is 0.010%, further preferably is0.012%, and further preferably is 0.015%. A preferable upper limit ofthe Nb content is 0.080%, more preferably is 0.060%, further preferablyis 0.050%, and further preferably is 0.030%.

B: 0.0005 to 0.0040% Boron (B) dissolves in the steel, increasing thehardenability of the steel material and increases the yield strength ofthe steel material. When the B content is too low, this advantageouseffect cannot be obtained. On the other hand, when the B content is toohigh, coarse nitrides are formed, and the SSC resistance of the steelmaterial is reduced. Accordingly, the B content is within a range of0.0005 to 0.0040%. A preferable lower limit of the B content is 0.0007%,more preferably is 0.0010%, and further preferably is 0.0012%. Apreferable upper limit of the B content is 0.0035%, more preferably is0.0030%, and further preferably is 0.0025%.

N: 0.0100% or Less

Nitrogen (N) is unavoidably contained. That is, the N content is morethan 0%. N combines with Ti to form fine nitrides and thereby refinesthe microstructure of the steel material by a pinning effect. As aresult, the SSC resistance of the steel material is increased. On theother hand, when the N content is too high, coarse nitrides are formed,and the SSC resistance of the steel material is reduced. Accordingly,the N content is 0.0100% or less. A preferable upper limit of the Ncontent is 0.0080%, and more preferably is 0.0070%. A preferable lowerlimit of the N content for effectively obtaining the aforementionedadvantageous effects is 0.0020%, more preferably 0.0025%, furtherpreferably is 0.0030%, further preferably is 0.0035%, and furtherpreferably is 0.0040%.

O: Less than 0.0020%

Oxygen (O) is an impurity. That is, the O content is more than 0%. Oforms coarse oxides and reduces the SSC resistance of the steelmaterial. Accordingly, the O content is less than 0.0020%. A preferableupper limit of the O content is 0.0018%, and more preferably is 0.0015%.Preferably, the O content is as low as possible. However, when the Ocontent is excessively reduced, the production cost increasessignificantly. Accordingly, in consideration of industrial production, apreferable lower limit of the O content is 0.0001%, and more preferablyis 0.0003%.

The balance of the chemical composition of the steel material accordingto the present embodiment is Fe and impurities. In the presentembodiment, “impurities” mean materials which are mixed into the steelmaterial from ore or scrap as a raw material, a production environmentor the like in industrially producing the steel material, and which areallowed within a range where the impurities do not adversely affect thesteel material of the present embodiment.

[Optional Element]

The chemical composition of the aforementioned steel material mayfurther contain one or more types of element selected from the groupconsisting of Ca, Mg, Zr, and rare earth metal (REM) in lieu of a partof Fe. Each of these elements is an optional element, that controls themorphology of sulfides in the steel material and increases the SSCresistance of the steel material.

Ca: 0 to 0.0100%

Calcium (Ca) is an optional element, and may not be contained. That is,the Ca content may be 0%. When Ca is contained, Ca renders S in thesteel material harmless by forming sulfides, and thereby increases theSSC resistance of the steel material. If even a small amount of Ca iscontained, it is possible to obtain this advantageous effect to someextent. However, when the Ca content is too high, oxides in the steelmaterial coarsen and the SSC resistance of the steel material isreduced. Accordingly, the Ca content is within a range of 0 to 0.0100%.A preferable lower limit of the Ca content is more than 0%, morepreferably is 0.0001%, further preferably is 0.0003%, further preferablyis 0.0006%, and further preferably is 0.0010%. A preferable upper limitof the Ca content is 0.0040%, more preferably is 0.0030%, furtherpreferably is 0.0025%, and further preferably is 0.0020%.

Mg: 0 to 0.0100%

Magnesium (Mg) is an optional element, and may not be contained. Thatis, the Mg content may be 0%. When Mg is contained, Mg renders S in thesteel material harmless by forming sulfides, and thereby increases theSSC resistance of the steel material. If even a small amount of Mg iscontained, it is possible to obtain this advantageous effect to someextent. However, when the Mg content is too high, oxides in the steelmaterial coarsen and the SSC resistance of the steel material isreduced. Accordingly, the Mg content is within a range of 0 to 0.0100%.A preferable lower limit of the Mg content is more than 0%, morepreferably is 0.0001%, further preferably is 0.0003%, further preferablyis 0.0006%, and further preferably is 0.0010%. A preferable upper limitof the Mg content is 0.0040%, more preferably is 0.0030%, furtherpreferably is 0.0025%, and further preferably is 0.0020%.

Zr: 0 to 0.0100%

Zirconium (Zr) is an optional element, and may not be contained. Thatis, the Zr content may be 0%. When Zr is contained, Zr renders S in thesteel material harmless by forming sulfides, and thereby increases theSSC resistance of the steel material. If even a small amount of Zr iscontained, it is possible to obtain this advantageous effect to someextent. However, when the Zr content is too high, oxides in the steelmaterial coarsen and the SSC resistance of the steel material isreduced. Accordingly, the Zr content is within a range of 0 to 0.0100%.A preferable lower limit of the Zr content is more than 0%, morepreferably is 0.0001%, further preferably is 0.0003%, further preferablyis 0.0006%, and further preferably is 0.0010%. A preferable upper limitof the Zr content is 0.0040%, more preferably is 0.0030%, furtherpreferably is 0.0025%, and further preferably is 0.0020%.

Rare Earth Metal (REM): 0 to 0.0100%

Rare earth metal (REM) is an optional element, and may not be contained.That is, the REM content may be 0%. When REM is contained, REM renders Sin the steel material harmless by forming sulfides, and therebyincreases the SSC resistance of the steel material. REM also combineswith P in the steel material and suppresses segregation of P at thegrain boundaries. Therefore, a reduction in the low temperaturetoughness and the SSC resistance of the steel material that isattributable to segregation of P is suppressed. If even a small amountof REM is contained, it is possible to obtain these advantageous effectsto some extent. However, when the REM content is too high, oxides in thesteel material coarsen, and the low temperature toughness and the SSCresistance of the steel material are reduced. Accordingly, the REMcontent is within a range of 0 to 0.0100%. A preferable lower limit ofthe REM content is more than 0%, more preferably is 0.0001%, furtherpreferably is 0.0003%, further preferably is 0.0006%, and furtherpreferably is 0.0010%. A preferable upper limit of the REM content is0.0040%, more preferably is 0.0030%, further preferably is 0.0025%, andfurther preferably is 0.0020%.

Note that, in the present description the term “REM” refers to one ormore types of element selected from a group consisting of scandium (Sc)which is the element with atomic number 21, yttrium (Y) which is theelement with atomic number 39, and the elements from lanthanum (La) withatomic number 57 to lutetium (Lu) with atomic number 71 that arelanthanoids. Further, in the present description the term “REM content”refers to the total content of these elements.

The chemical composition of the aforementioned steel material mayfurther contain one or more types of element selected from the groupconsisting of Cu and Ni in lieu of a part of Fe. Each of these elementsis an optional element, and increases the hardenability of the steelmaterial.

Cu: 0 to 0.50%

Copper (Cu) is an optional element, and may not be contained. That is,the Cu content may be 0%. When Cu is contained, Cu increases thehardenability of the steel material, and thereby increasing the yieldstrength of the steel material. If even a small amount of Cu iscontained, it is possible to obtain this advantageous effect to someextent. However, when the Cu content is too high, the hardenability ofthe steel material becomes too high, and the SSC resistance of the steelmaterial is reduced. Accordingly, the Cu content is within a range of 0to 0.50%. A preferable lower limit of the Cu content is more than 0%,more preferably is 0.02%, further preferably is 0.03%, and furtherpreferably is 0.05%. A preferable upper limit of the Cu content is0.35%, and more preferably is 0.25%.

Ni: 0 to 0.50%

Nickel (Ni) is an optional element, and may not be contained. That is,the Ni content may be 0%. When Ni is contained, Ni increases thehardenability of the steel material, and thereby increasing the yieldstrength of the steel material. If even a small amount of Ni iscontained, it is possible to obtain this advantageous effect to someextent. However, when the Ni content is too high, corrosion is locallypromoted, and thereby the SSC resistance of the steel material isreduced. Accordingly, the Ni content is within a range of 0 to 0.50%. Apreferable lower limit of the Ni content is more than 0%, morepreferably is 0.02%, further preferably is 0.03%, and further preferablyis 0.05%. A preferable upper limit of the Ni content is 0.35%, and morepreferably is 0.25%.

The chemical composition of the aforementioned steel material mayfurther contain one or more types of element selected from the groupconsisting of Co and W in lieu of a part of Fe. Each of these elementsis an optional element, that forms corrosion coating havingprotectability in a hydrogen sulfide environment, and therebysuppressing hydrogen penetration. With such a configuration, theseelements increase the SSC resistance of the steel material.

Co: 0 to 0.50%

Cobalt (Co) is an optional element, and may not be contained. That is,the Co content may be 0%. When Co is contained, Co forms corrosioncoating having protectability in a hydrogen sulfide environment, andthereby suppressing hydrogen penetration. As a result, the SSCresistance of the steel material is increased. If even a small amount ofCo is contained, it is possible to obtain this advantageous effect tosome extent. However, when the Co content is too high, the hardenabilityof the steel material is reduced so that the yield strength of the steelmaterial is reduced. Accordingly, the Co content is within a range of 0to 0.50%. A preferable lower limit of the Co content is more than 0%,more preferably is 0.02%, further preferably is 0.03%, and furtherpreferably is 0.05%. A preferable upper limit of the Co content is0.45%, and more preferably is 0.40%.

W: 0 to 0.50%

Tungsten (W) is an optional element, and may not be contained. That is,the W content may be 0%. When W is contained, W forms corrosion coatinghaving protectability in a hydrogen sulfide environment, and therebysuppressing hydrogen penetration. As a result, the SSC resistance of thesteel material is increased. If even a small amount of W is contained,it is possible to obtain this advantageous effect to some extent.However, when the W content is too high, coarse carbides are formed inthe steel material, and the SSC resistance of the steel material isreduced. Accordingly, the W content is within a range of 0 to 0.50%. Apreferable lower limit of the W content is more than 0%, more preferablyis 0.02%, further preferably is 0.03%, and further preferably is 0.05%.A preferable upper limit of the W content is 0.45%, and more preferablyis 0.40%.

[Formula (1)]

The chemical composition of the steel material according to the presentembodiment also satisfies Formula (1).

2.7×C+0.4×Si+Mn+0.45×Ni+0.45×Cu+0.8×Cr+2×Mo≥3.90  (1)

where, content (mass %) of a corresponding element is substituted foreach symbol of an element in Formula (1), and if a corresponding elementis not contained, “0” is substituted for the element symbol of therelevant element.

F1 (=2.7×C+0.4×Si+Mn+0.45×Ni+0.45×Cu+0.8×Cr+2×Mo) is an index showingthe hardenability of the steel material having the aforementionedchemical composition. When F1 is less than 3.90, sufficienthardenability cannot be obtained, and the yield strength of the steelmaterial cannot be obtained. Accordingly, the steel material accordingto the present embodiment has F1 of 3.90 or more.

A preferable lower limit of F1 is 3.93, and more preferably is 4.00. Apreferable upper limit of F1 is not particularly limited. However, inthe steel material according to the present embodiment having theaforementioned chemical composition, the upper limit of F1 may be 8.27,for example. A preferable upper limit of F1 is 8.20, more preferably is8.10, and further preferably is 8.00.

[Prior-Austenite Grain Diameter]

In the microstructure of the steel material according to the presentembodiment, the prior-austenite grain diameter (prior γ grain diameter)is 11.0 μm or less. As described above, in the present description, thegrain diameter of prior-austenite grain (prior γ grain diameter) meansthe grain diameter of prior-austenite grain obtained in accordance witha comparison method of ASTM E112-10. When prior γ grain of a steelmaterial is fine, yield strength and SSC resistance are stablyincreased. In view of the above, in the present embodiment, the steelmaterial contains Mo of more than 1.00% to make prior γ grain of thesteel material fine.

When the prior γ grain diameter in the steel material according to thepresent embodiment is 11.0 μm or less, both yield strength of 110 ksigrade and excellent SSC resistance can be achieved provided that theother specifications of the steel material according to the presentembodiment are satisfied.

A preferable upper limit of the prior γ grain diameter in the steelmaterial according to the present embodiment is 10.5 μm, and morepreferably is 10.0 μm. A preferable lower limit of the prior γ graindiameter in the steel material according to the present embodiment isnot particularly limited. However, the lower limit of the prior γ graindiameter in the steel material according to the present embodiment maybe 4.5 μm, for example.

As described above, the prior γ grain diameter can be obtained inaccordance with a comparison method of ASTM E112-10. More specifically,the prior γ grain diameter can be acquired by the following method. Inthe case where the steel material is a steel plate, a test specimenhaving an observation surface perpendicular to the rolling direction iscut out from the center portion of the thickness. In the case where thesteel material is a steel pipe, a test specimen having an observationsurface perpendicular to the axial direction of the steel pipe is cutout from the center portion of the wall thickness. The observationsurface is polished into a mirror surface and, thereafter, is embeddedinto a resin. Then, the test specimen is immersed into a 2% nitaletching reagent for approximately 10 seconds to develop prior γ grainboundaries by etching.

The etched observation surface is subjected to 10 field observation in asecondary electron image using a Scanning Electron Microscope (SEM) toform a photographic image. The observation magnification is ×200, forexample. By comparing the formed photographic image with a grain sizenumber standard view which is defined in ASTM E112-10, the grain sizenumber is evaluated. The average grain diameter of prior γ grain in eachvisual field is acquired from the evaluated grain size number. Thearithmetic average value of the average grain diameters of prior γgrains acquired in 10 visual field is defined as the grain diameter ofthe prior γ grain (prior γ grain diameter) (μm).

[Precipitates which are Precipitated in Prior γ Grain Boundaries]

In the steel material according to the present embodiment, the averagearea of precipitates which are precipitated in the prior-austenite grainboundaries (prior γ grain boundaries) is 10.0×10⁻³ μm² or less. In thepresent description, precipitates which are precipitated in the prior γgrain boundaries are also referred to as “specific precipitates”. Whenthe average area of specific precipitates is 10.0×10⁻³ μm² or less, bothyield strength of 110 ksi grade and excellent SSC resistance can beachieved provided that the other specifications of the steel materialaccording to the present embodiment are satisfied.

As described above, in the steel material having the aforementionedchemical composition and the prior γ grain diameter of 11.0 μm or less,when an attempt is made to obtain yield strength of 110 ksi grade, theremay be a case where a large amount of coarse carbide is precipitated inthe steel material. Further, of the coarse carbide in the steelmaterial, carbide which is precipitated in the prior γ grain boundariesreduces SSC resistance of the steel material. In the steel materialaccording to the present embodiment, most of the precipitates which areprecipitated in the prior γ grain boundaries are carbide.

In view of the above, in the steel material according to the presentembodiment, the average area of precipitates (specific precipitates)which are precipitated in the prior γ grain boundaries is set to10.0×10⁻³ μm² or less. When the average area of the specificprecipitates is more than 10.0×10⁻³ μm², there may be a case where SSCresistance of a steel material be reduced. When the average area of thespecific precipitates is more than 10.0×10⁻³ μm², there may be also acase where yield strength of 758 to 862 MPa (110 ksi grade) cannot beobtained.

Accordingly, in the steel material according to the present embodiment,the average area of precipitates which are precipitated in the prior γgrain boundaries is 10.0×10⁻³ μm² or less. A preferable upper limit ofthe average area of the specific precipitates is 9.9×10⁻³ μm², and morepreferably is 9.7×10⁻³ μm.

The lower limit of the average area of the specific precipitates is notparticularly limited, and may be 0.0×10⁻³ μm². However, in the steelmaterial according to the present embodiment having the aforementionedchemical composition, the lower limit of the average area of thespecific precipitates may be 3.0×10⁻³ μm², for example.

The average area of the specific precipitates can be acquired by thefollowing method. A test specimen is cut out from the steel material ina similar manner of the aforementioned determined method of the prior γgrain diameter. Specifically, in the case where the steel material is asteel plate, a test specimen having an observation surface perpendicularto the rolling direction is cut out from the center portion of thethickness. In the case where the steel material is a steel pipe, a testspecimen having an observation surface perpendicular to the axialdirection of the steel pipe is cut out from the center portion of thewall thickness. The observation surface is polished into a mirrorsurface and, thereafter, is embedded into a resin. Then, the testspecimen is immersed into a 2% vital etching reagent for approximately10 seconds to develop prior γ grain boundaries by etching. The etchedobservation surface is subjected to 10 field observation in a secondaryelectron image using a SEM to form a photographic image. The observationmagnification is ×10000 (ten thousand), for example.

The prior γ grain boundaries are specified from the formed photographicimage based on the contrast. The precipitates are also specified fromthe formed photographic image based on the contrast. Note that, asdescribed above, the observation magnification is ×10000, for example.In addition, precipitates can be identified based on contrast when theprecipitates have the equivalent circular diameter is 50 nm or more. Onthe other hands, in the present embodiment, the upper limit ofequivalent circular diameter of the identified precipitates is notparticularly limited. In the steel material having the aforementionedchemical composition, the upper limit of the equivalent circulardiameter of the identified precipitates is 1000 nm, for example.Therefore, in the present embodiment, the equivalent circular diameterof the identified precipitates is within a range of 50 to 1000 nm, forexample.

Precipitates which overlap with the specified prior γ grain boundariesand/or which come into contact with the specified prior γ grainboundaries are specified as “specific precipitates”. That is, thespecific precipitates (precipitates which are precipitated in the priorγ grain boundaries) mean precipitates which partially overlap and/orcome into contact with the prior γ grain boundary. The average area(μm²) of the specified specific precipitates is acquired by performingan image analysis.

[Microstructure]

The microstructure of the steel material according to the presentembodiment is principally composed of tempered martensite and temperedbainite. More specifically, in the microstructure, the sum of the volumeratio of tempered martensite and the volume ratio of tempered bainite is90% or more. The balance of microstructure consists of ferrite orpearlite, for example.

When the microstructure of the steel material having the aforementionedchemical composition contains tempered martensite and tempered bainitesuch that the sum of the volume ratio of tempered martensite and thevolume ratio of tempered bainite is 90% or more, the steel material hasyield strength of 758 to 862 MPa (110 ksi grade) provided that the otherspecifications of the present embodiment are satisfied.

The sum of the volume ratio of tempered martensite and the volume ratioof tempered bainite can be acquired by performing the microstructureobservation. In performing the microstructure observation, theaforementioned photographic image formed at the time of acquiring theprior γ grain diameter is used. In each visual field, temperedmartensite and tempered bainite can be distinguished from other phases(ferrite or pearlite, for example) based on the contrast. Accordingly,in each visual field, tempered martensite and tempered bainite arespecified based on the contrast.

The sum of the area fraction of the specified tempered martensite andthe area fraction of the specified tempered bainite is acquired. In thepresent embodiment, the arithmetic average value of the sums of the areafraction of tempered martensite and the area fraction of temperedbainite, which are acquired in all visual fields, is assumed as thevolume ratio of tempered martensite and tempered bainite.

[Yield Strength of Steel Material]

Yield strength of the steel material according to the present embodimentis 758 to 862 MPa (110 ksi grade). Yield strength in the presentdescription means stress at 0.7% elongation (0.7% yield stress) acquiredin a tensile test. Even if yield strength of the steel materialaccording to the present embodiment is 110 ksi grade, the steel materialaccording to the present embodiment has excellent SSC resistanceprovided that the aforementioned chemical composition, prior γ graindiameter, and average area of the specific precipitates are satisfied.

Yield strength of the steel material according to the present embodimentcan be acquired by the following method. The tensile test is performedby a method conforming to ASTM E8/E8M (2013). A round bar test specimenis taken from the steel material according to the present embodiment. Inthe case where the steel material is a steel plate, a round bar testspecimen is taken from a center portion of the thickness. In the casewhere a steel material is a steel pipe, a round bar test specimen istaken from a center portion of the wall thickness. The size of the roundbar test specimen is such that the diameter of a parallel portion is 8.9mm and the length of the parallel portion is 35.6 mm, for example. Theaxial direction of the round bar test specimen is parallel to therolling direction of the steel material. The tensile test is performedusing the round bar test specimen in the atmosphere at the normaltemperature (25° C.) and the acquired stress at 0.7% elongation isdefined as yield strength (MPa).

[SSC Resistance of Steel Material]

SSC resistance of the steel material according to the present embodimentcan be evaluated by a method in accordance with “Method A” specified inNACE TM0177-2005, and a four-point bending test.

In the method in accordance with “Method A” specified in NACETM0177-2005, a round bar test specimen is taken from the steel materialaccording to the present embodiment. In the case where the steelmaterial is a steel plate, a round bar test specimen is taken from acenter portion of the thickness. In the case where the steel material isa steel pipe, a round bar test specimen is taken from the center portionof the wall thickness. The size of the round bar test specimen is suchthat a diameter is 6.35 mm, and the length of a parallel portion is 25.4mm, for example. The axial direction of the round bar test specimen isparallel to the rolling direction of the steel material.

A mixed aqueous solution containing 5.0 mass % of sodium chloride and0.5 mass % of acetic acid (NACE solution A) at 4° C. is employed as atest solution. A stress equivalent to 90% of the actual yield stress isapplied to the round bar test specimen. The test solution at 4° C. ispoured into a test vessel so that the round bar test specimen to whichthe stress has been applied is immersed therein, and this is adopted asa test bath. After degassing the test bath, H₂S gas at 1 atm pressure isblown into the test bath and is caused to saturate in the test bath. Thetest bath where saturation of the H₂S gas is caused is held for 720hours at 4° C.

On the other hand, in the four-point bending test, a test specimen istaken from the steel material according to the present embodiment. Inthe case where the steel material is a steel plate, the test specimen istaken from a center portion of the thickness. In the case where thesteel material is a steel pipe, the test specimen is taken from thecenter portion of the wall thickness. The size of the test specimen issuch that the thickness is 2 mm, a width is 10 mm, and a length is 75mm, for example. The length direction of the test specimen is parallelto the rolling direction of the steel material.

An aqueous solution containing 5.0 mass % of sodium chloride at 24° C.is employed as the test solution. In accordance with ASTM G39-99 (2011),stress is applied to the test specimens by four-point bending so thatthe stress applied to each test specimen becomes 90% of the actual yieldstress. The test specimen to which stress has been applied is enclosedin an autoclave, together with the test jig. The test solution is pouredinto the autoclave in a manner so as to leave a vapor phase portion, andadopted as the test bath. After the test bath is degassed, 20 atm H₂Sgas is sealed under pressure in the autoclave, and the test bath isstirred to cause the H₂S gas to saturate. After sealing the autoclave,the test bath is stirred for 720 hours at 24° C.

In the steel material according to the present embodiment, cracking isnot confirmed after 720 hours elapses in both the method in accordancewith “Method A” and the four-point bending test. Note that, in thepresent description, the term “cracking is not confirmed” means thatcracking is not confirmed in a test specimen in a case where the testspecimen after the test was observed by the naked eye.

[Shape of Steel Material]

The shape of the steel material according to the present embodiment isnot particularly limited. The steel material may be a steel pipe or asteel plate, for example. In the case where the steel material is anoil-well steel pipe, a preferable wall thickness is 9 to 60 mm. Morepreferably, the steel material according to the present embodiment issuitable for use as a heavy-wall seamless steel pipe. More specifically,even when the steel material according to the present embodiment is aseamless steel pipe having a wall thickness of 15 mm or more or,furthermore, 20 mm or more, the steel material exhibits the yieldstrength of 110 ksi grade and excellent SSC resistance.

[Production Method]

A method for producing the steel material according to the presentembodiment will be described. The production method describedhereinafter is a method for producing a seamless steel pipe, which isone example of the steel material according to the present embodiment.Note that the method for producing the steel material according to thepresent embodiment is not limited to the production method which will bedescribed hereinafter.

[Preparing Process]

In a preparing process, an intermediate steel material having theaforementioned chemical composition is prepared. Provided that theintermediate steel material has the aforementioned chemical composition,a method for producing the intermediate steel material is notparticularly limited. In the present embodiment, in the case where anend product is a steel plate, the intermediate steel material is aplate-shaped steel material. Meanwhile in the case where the end productis a steel pipe, the intermediate steel material is a hollow shell.

The preparing process may preferably include a process of preparing astarting material (starting material preparing process), and a processof producing an intermediate steel material by performing hot working onthe starting material (hot working process). Hereinafter, the case wherethe preparing process includes the starting material preparing processand the hot working process will be described in detail.

[Starting Material Preparing Process]

In the starting material preparing process, a starting material isproduced using molten steel having the aforementioned chemicalcomposition. Specifically, a cast piece (slab, bloom, or billet) isproduced by a continuous casting process using molten steel. An ingotmay be produced by an ingot-making process using molten steel. A billetmay be produced by blooming a slab, bloom or ingot when necessary. Thestarting material (slab, bloom, or billet) is produced via theaforementioned processes.

[Hot Working Process]

In the hot working process, hot working is performed on the preparedstarting material, thus producing an intermediate steel material. In thecase where the steel material is a steel pipe, the intermediate steelmaterial corresponds to a hollow shell. First, a billet is heated in aheating furnace. The heating temperature is not particularly limited,for example, the heating temperature may be 1100 to 1300° C. Hot workingis performed on the billet extracted from the heating furnace to producea hollow shell (seamless steel pipe).

For example, the Mannesmann process may be performed for hot working toproduce a hollow shell. In this case, a round billet is subject topiercing-rolling by a piercing machine. In the case of performingpiercing-rolling, a piercing ratio is not particularly limited, forexample, the piercing ratio may be 1.0 to 4.0. The round billet on whichpiercing-rolling is performed is further subject to hot rolling by amandrel mill, a reducer, a sizing mill or the like, thus forming ahollow shell. The cumulative reduction of area in the hot workingprocess is, for example, 20 to 70%.

A hollow shell may be produced from a billet by another hot workingmethod. For example, in the case of a heavy-wall steel material having ashort length, such as coupling, a hollow shell may be produced byperforming forging by the Ehrhardt method or the like. The hollow shellis produced via the aforementioned processes. The wall thickness of ahollow shell to be produced is not particularly limited, for example,the wall thickness may be 9 to 60 mm.

The hollow shell produced by hot working may be air-cooled (As-Rolled).The hollow shell produced by hot working may be subjected to directquenching after hot working without being cooled to normal temperature,or may be subjected to quenching after undergoing supplementary heating(reheating) after hot working.

In the case where direct quenching is performed or quenching isperformed after supplementary heating is performed, stopping of coolingor slow cooling may be performed during quenching. In this case, it ispossible to suppress the occurrence of quenching cracks in the hollowshell. In the case where direct quenching is performed or quenching isperformed after supplementary heating is performed, a stress relieftreatment (SR treatment) may be further performed after quenching andbefore heat treatment (tempering or the like) which is a next process.In this case, residual stress in the hollow shell is removed.

As described above, the intermediate steel material is prepared in thepreparing process. The intermediate steel material may be produced bythe aforementioned preferred processes, or may be an intermediate steelmaterial produced by a third party, or an intermediate steel materialthat was produced in another factory other than the factory where aquenching process and a tempering process described later are performed,or at a different works.

[Heat Treatment Process]

In the heat treatment process, heat treatment is performed on theprepared intermediate steel material. Specifically, quenching andtempering are performed on the prepared intermediate steel material. Inthe present description, “quenching” means to rapidly cool anintermediate steel material at the temperature of the A₃ point or more.In the present description, “tempering” means to reheat and hold thequenched intermediate steel material at the temperature of the A_(c1)point or less.

In the heat treatment process according to the present embodiment, it ispreferable to perform quenching and tempering a plurality of times.Specifically, it is preferable to perform each of quenching andtempering two or more times. More specifically, it is preferable thatquenching is performed and, thereafter, tempering is performed on theprepared intermediate steel material. Further, quenching is performedand, then, tempering is performed on the prepared intermediate steelmaterial.

Note that, in the heat treatment process according to the presentembodiment, quenching and tempering may be performed three or moretimes. However, even if quenching and tempering are repeatedly performedfour or more times, the advantageous effects obtained by performing theheat treatment saturates. Accordingly, in the heat treatment processaccording to the present embodiment, it is preferable to performquenching and tempering two or three times. Hereinafter, quenching andtempering will be described in detail.

[Quenching]

Quenching is performed on the prepared intermediate steel material(hollow shell) and/or the intermediate steel material on which temperingis performed. In the heat treatment process according to the presentembodiment, a preferred quenching temperature is 800 to 1000° C. In thepresent description, “quenching temperature” corresponds to the surfacetemperature of the intermediate steel material measured by a thermometerinstalled on the exit side of an apparatus which performs final hotworking in the case where direct quenching is performed after hotworking is performed. The quenching temperature also corresponds to atemperature of a supplementary heating furnace or a heat treatmentfurnace in the case where quenching is performed using the holdingfurnace or the heat treatment furnace after hot working is performed.

That is, in the heat treatment process according to the presentembodiment, quenching may be performed by rapidly cooling theintermediate steel material at 800 to 1000° C. after hot working isperformed. Quenching may be performed such that the intermediate steelmaterial on which hot working is performed is heated to 800 to 1000° C.using the supplementary heating furnace or the heat treatment furnaceand, then, is rapidly cooled. Alternatively, quenching may be performedsuch that the intermediate steel material on which tempering isperformed is heated to 800 to 1000° C. using the heat treatment furnaceand, then, is rapidly cooled.

When the quenching temperature is too high, there may be a case whereprior γ grain is coarsened, thus reducing SSC resistance of a steelmaterial. Accordingly, quenching temperature is preferably set to 800 to1000° C. A more preferable upper limit of the quenching temperature is950° C.

In the heat treatment process according to the present embodiment, inthe case where quenching is performed using the supplementary heatingfurnace or the heat treatment furnace after hot working is performed, apreferred quenching time is 5 to 20 minutes. In the present description,“quenching time” means a time from a point of time when an intermediatesteel material is charged into the supplementary heating furnace or theheat treatment furnace to a point of time when the intermediate steelmaterial is taken out.

In the case where quenching is performed using the supplementary heatingfurnace or the heat treatment furnace after hot working is performed, ifthe quenching time is too long, prior γ grain may be coarsened afterlast tempering is performed. Accordingly, in the case where quenching isperformed using the supplementary heating furnace or the heat treatmentfurnace after hot working is performed in the heat treatment processaccording to the present embodiment, it is preferable to set thequenching time to 5 to 20 minutes.

For example, a quenching method may be adopted where a hollow shell iscontinuously cooled from a temperature at which quenching is started tocontinuously reduce the temperature of the hollow shell. The method fora continuous cooling process is not particularly limited, and awell-known method may be adopted. The method for the continuous coolingprocess may be a method where a hollow shell is immersed into a watertank to cool, or a method where a hollow shell is cooled by shower wateror is cooled by mist to perform accelerated cooling.

When a cooling speed during quenching is too low, a microstructure whichis principally composed of martensite and bainite cannot be obtained sothat mechanical property which is defined in the present embodimentcannot be obtained. Accordingly, in the method for producing a steelmaterial according to the present embodiment, an intermediate steelmaterial (hollow shell) is rapidly cooled during quenching.Specifically, in the quenching process, an average cooling speed whenthe temperature of the intermediate steel material (hollow shell) duringquenching falls within a range of 800 to 500° C. is defined as a coolingspeed during quenching CR₈₀₀₋₅₀₀ (° C./sec). More specifically, thecooling speed during quenching CR₈₀₀₋₅₀₀ is decided from a temperaturemeasured on the surface of the quenched intermediate steel material.

A preferred cooling speed during quenching CR₈₀₀₋₅₀₀ is 8° C./sec ormore. In this case, the microstructure of an intermediate steel material(hollow shell) on which quenching is performed is principally composedof martensite and bainite in a stable manner. A preferable lower limitof the cooling speed during quenching CR₈₀₀₋₅₀₀ is 10° C./sec. Apreferable upper limit of the cooling speed during quenching CR₈₀₀₋₅₀₀is 500° C./sec.

[Tempering]

Tempering is performed on the intermediate steel material on which theaforementioned quenching is performed. In performing tempering on asteel material which is expected to be used in a sour environment, atempering temperature and a tempering time are adjusted according to thechemical composition of the steel material and yield strength which isexpected to be obtained. In this case, only last tempering is controlledand, conventionally, it is considered sufficient to set a temperingtemperature to A_(c1) point or less during tempering other than lasttempering.

On the other hand, in the steel material according to the presentembodiment, prior γ grain is made fine by increasing Mo content. Withrespect to this mechanism, as described above, it is considered that thedissolved Mo in the steel material segregates in austenite grainboundaries during heating in a quenching process, thus making prior γgrain after tempering fine by a pinning effect. In the presentembodiment, Mo is liable to form M₂C carbide in the steel materialhaving the aforementioned chemical composition. Further, in the steelmaterial having the aforementioned chemical composition, M₂C carbide isliable to be precipitated during tempering.

In view of the above, in the heat treatment process according to thepresent embodiment, the sufficient amount of Mo is dissolved in a steelmaterial on which second last tempering is performed. Specifically, inthe heat treatment process according to the present embodiment, atempering parameter TMP₂ (=(tempering temperature (° C.)+273)×(log(tempering time (min)/60)+20)) is controlled during second lasttempering, and thereby it is possible to reduce the amount of Mo whichis precipitated as M₂C carbide.

More specifically, in the steel material having the aforementionedchemical composition, when the tempering parameter TMP₂ during thesecond last tempering is 15000 to 19000, it is possible to make theprior γ grain diameter in the steel material on which last tempering isperformed fine. When the tempering parameter TMP₂ during the second lasttempering is less than 15000, there may be a case where advantageouseffects of tempering cannot be sufficiently obtained so that quenchingcracks or season cracks occur in the steel material. On the other hand,when the tempering parameter TMP₂ during the second last tempering ismore than 19000, there may be a case where the sufficient amount ofdissolved Mo cannot be obtained during heating in the last quenching sothat a prior γ grain on which last tempering is performed is coarsened.

Accordingly, in the heat treatment process according to the presentembodiment, a preferable tempering parameter TMP₂ during the second lasttempering is 15000 to 19000. A more preferable lower limit of thetempering parameter TMP₂ during the second last tempering is 15500, andfurther preferably is 16000. A more preferable upper limit of thetempering parameter TMP₂ during second last tempering is 18500, andfurther preferably is 18000.

In the second last tempering, a preferable tempering temperature is 500to less than 700° C. In the second last tempering, a more preferabletempering time (holding time) is 10 to 60 minutes. That is, in thepresent embodiment, in the second last tempering, the temperingtemperature is set to 500 to less than 700° C., and the tempering timeis set to 10 to 60 minutes, and further, the tempering parameter TMP₂ isset to 15000 to 19000.

Note that, “tempering temperature” in the present descriptioncorresponds to a temperature of a heat treatment furnace at the time ofheating and holding an intermediate steel material on which quenching isperformed. In the present description, a tempering time (holding time)means a time from a point of time when the intermediate steel materialis charged into the heat treatment furnace for heating and holding theintermediate steel material on which quenching is performed to a pointof time when the intermediate steel material is taken out.

Further, in the present description, “second last tempering” meanstempering performed before last quenching and tempering. That is, in thecase where each of quenching and tempering is performed two times in theheat treatment process, second last tempering means the first tempering.In the case where each of quenching and tempering is performed threetimes in the heat treatment process, second last tempering means thesecond tempering.

The steel material according to the present embodiment further reducescoarse specific precipitates of precipitates which are precipitated inthe prior γ grain boundaries (specific precipitates). As describedabove, most of the specific precipitates are carbide. Therefore, most ofthe specific precipitates are precipitated in last tempering.Accordingly, in the heat treatment process according to the presentembodiment, not only the tempering parameter TMP₂ during second lasttempering, but also a tempering parameter TMP₁ during last tempering(=(tempering temperature (° C.)+273)×(log (tempering time (min)/60)+20))are controlled.

More specifically, in the steel material having the aforementionedchemical composition, provided that the tempering parameter TMP₁ duringthe last tempering is 19100 to 19600, coarse specific precipitates canbe reduced in the steel material on which last tempering is performed.When the tempering parameter TMP₁ during last tempering is less than19100, there may be a case where advantageous effects of temperingcannot be sufficiently obtained, and yield strength of a steel materialon which tempering is performed becomes too high. When the temperingparameter TMP₁ during last tempering is less than 19100, there may bealso a case where a large amount of coarse specific precipitates isprecipitated.

On the other hand, when the tempering parameter TMP₁ during lasttempering is more than 19600, there may be a case where yield strengthof a steel material on which tempering is performed becomes too low.When the tempering parameter TMP₁ during last tempering is more than19600, there may be also a case where a large amount of coarse specificprecipitates is precipitated.

Accordingly, in the heat treatment process according to the presentembodiment, a preferable tempering parameter TMP₁ during the lasttempering is 19100 to 19600. A more preferable lower limit of thetempering parameter TMP₁ during last tempering is 19200, and furtherpreferably is 19300. A more preferable upper limit of the temperingparameter TMP₁ during last tempering is 19570, and further preferably is19500.

In the last tempering, a preferable tempering temperature is 650 to 730°C. In the last tempering, a preferable tempering time (holding time) is10 to 90 minutes. That is, in the present embodiment, in the lasttempering, the tempering temperature is set to 650 to 730° C., and thetempering time is set to 10 to 90 minutes and further, the temperingparameter TMP₁ is set to 19100 to 19600.

In the case where the steel material is a steel pipe, variation isliable to occur in temperature of the steel pipe during holding intempering compared with another shape. Accordingly, in the case wherethe steel material is a steel pipe, a preferable tempering time is 15 to90 minutes. It is sufficiently possible for those skilled in the art toset yield strength to 758 to 862 MPa (110 ksi grade) by appropriatelyadjusting the aforementioned tempering temperature and theaforementioned tempering time of the steel material having the chemicalcomposition of the present embodiment.

The steel material according to the present embodiment can be producedby the aforementioned production method. In the aforementionedproduction method, the method for producing a seamless steel pipe hasbeen described as one example. However, the steel material according tothe present embodiment may be a steel plate, or may have another shape.In the same manner as the aforementioned production method, the methodfor producing a steel plate or a product having another shape alsoincludes a preparing process and a heat treatment process, for example.Further, the aforementioned production method merely forms one example,and the steel material may be produced by another production method.

Examples

Molten steels having the chemical composition shown in Table 1 wereproduced. F1 for each steel was also acquired from the chemicalcomposition described in Table 1. Note that “-” in Table 1 means thatcontent of each element is at the level of an impurity.

TABLE 1 Chemical composition (unit being mass %, balance being Fe andimpurities) Steel C Si Mn P S Cr Mo Al N Ti Nb V A 0.27 0.27 0.45 0.0080.0009 0.75 1.25 0.027 0.0042 0.004 0.028 0.09 B 0.25 0.27 0.45 0.0080.0008 0.85 1.42 0.028 0.0035 0.007 0.025 0.09 C 0.27 0.33 0.25 0.0080.0010 0.76 1.50 0.029 0.0035 0.006 0.025 0.09 D 0.25 0.22 0.35 0.0080.0012 0.75 1.15 0.035 0.0035 0.006 0.027 0.09 E 0.26 0.24 0.45 0.0080.0011 0.76 1.22 0.035 0.0035 0.006 0.027 0.09 F 0.27 0.35 0.35 0.0080.0010 0.65 1.10 0.032 0.0035 0.006 0.027 0.09 G 0.28 0.21 0.45 0.0080.0010 0.76 1.49 0.032 0.0035 0.006 0.027 0.09 H 0.27 0.22 0.45 0.0080.0012 0.45 1.15 0.035 0.0035 0.006 0.027 0.10 I 0.27 0.22 0.45 0.0080.0013 0.50 0.30 0.035 0.0035 0.006 0.027 0.10 J 0.26 0.22 0.45 0.0070.0015 0.67 1.30 0.032 0.0045 0.004 0.026 0.09 K 0.27 0.30 0.45 0.0080.0012 1.05 0.30 0.035 0.0035 0.006 0.027 0.10 L 0.25 0.27 0.25 0.0070.0011 1.65 1.18 0.027 0.0033 0.004 0.025 0.09 M 0.26 0.35 0.55 0.0070.0013 1.48 0.77 0.025 0.0033 0.006 0.025 0.09 N 0.25 0.27 0.25 0.0070.0012 1.35 2.50 0.035 0.0032 0.006 0.026 0.11 O 0.27 0.28 0.45 0.0080.0010 0.82 1.20 0.035 0.0032 0.006 0.026 — Steel B O Ca Mg Zr REM Cu NiCo W F1 A 0.0012 0.0010 — — — — — — — — 4.39 B 0.0012 0.0011 0.0012 — —— — — — — 4.75 C 0.0012 0.0010 — — — — 0.05 — — — 4.74 D 0.0012 0.0011 —— — — — — 0.50 — 4.01 E 0.0012 0.0011 — 0.0011 — — — 0.05 — — 4.32 F0.0012 0.0011 — — 0.0011 — — — — 0.50 3.94 G 0.0012 0.0011 — — — — 0.03— 0.50 — 4.89 H 0.0012 0.0009 — — — — — — — — 3.93 I 0.0012 0.0009 — — —— — — — — 2.27 J 0.0011 0.0055 — — — — — — — — 4.38 K 0.0012 0.0009 — —— — — — — — 2.74 L 0.0011 0.0008 — — — — — — — — 4.71 M 0.0011 0.0008 —— — — — — — — 4.12 N 0.0013 0.0011 — — — — — — — — 7.11 O 0.0013 0.0011— — — — — — — — 4.35

Billets were produced using the aforementioned molten steels by acontinuous casting process. The produced billets of respective testnumbers were held for one hour at 1250° C., and thereafter hot rolling(hot working) was performed on the billets by the Mannesmann-mandrelmethod to produce hollow shells (seamless steel pipes) of respectivetest numbers.

Heat treatment (quenching and tempering) was performed two times on eachof the hollow shells of respective test numbers on which hot working wasperformed. Specifically, heat treatment was performed on the hollowshells of respective test numbers by the following method.

The hollow shells of respective test numbers produced by performing hotworking were held for 5 minutes in a supplementary heating furnace at950° C., and thereafter direct quenching (that is, first quenching) wasperformed. All of cooling speeds during quenching CR₈₀₀₋₅₀₀ in firstquenching for respective test numbers were within a range of 8 to 500°C./sec. Note that the cooling speed during quenching CR₈₀₀₋₅₀₀ wasacquired by measuring the surface temperature of the hollow shell ofeach test number.

Subsequently, first tempering, that is, second last tempering wasperformed on the hollow shells of respective test numbers. Specifically,on the hollow shell of each test number, tempering was performed whereeach hollow shell is held at the tempering temperature (° C.) for thetempering time (min) described in the column of “second last tempering”in Table 2. The tempering parameters TMP₂ (=(tempering temperature (°C.)+273)×(log (tempering time (min)/60)+20)) during second lasttempering are also shown in Table 2.

TABLE 2 Second last tempering Last quenching Last tempering Temper-Quench- Temper- Average ing Temper- ing Quench- ing Temper- Prior areatemper- ing temper- ing temper- ing γ grain of specific SSC resistanceTest ature time ature time ature time YS TS diameter precipitates 1 atm20 atm No. Steel (° C.) (min) TMP₂ (° C.) (min) (° C.) (min) TMP₁ (MPa)(MPa) (μm) (×10⁻³ μm²) H₂S H₂S  1 A 600 30 17197 920 10 700 60 19460 779857 7.3 6.0 E E  2 A 600 30 17197 920 10 700 60 19460 786 869 10.0 7.1 EE  3 B 600 30 17197 900 10 700 60 19460 800 872 9.0 6.1 E E  4 C 620 3017591 900 15 700 60 19460 793 863 6.6 8.1 E E  5 D 600 30 17197 900 10700 30 19167 827 913 5.5 8.9 E E  6 E 600 30 17197 920 10 705 60 19560772 835 9.6 6.7 E E  7 F 575 30 16705 920 15 700 60 19460 793 866 7.08.3 E E  8 G 575 30 16705 920 10 695 60 19360 814 883 8.1 5.1 E E  9 A700 30 19167 900 10 700 60 19460 793 871 13.1 6.2 NA E 10 B 700 30 19167900 10 700 45 19338 786 854 14.6 6.3 NA E 11 A 575 30 16705 920 10 71060 19660 745 807 8.4 12.6 E E 12 H 580 30 16803 900 10 700 60 19460 800869 13.1 6.1 E NA 13 I 600 30 17197 920 15 650 60 18460 786 869 18.612.5 NA NA 14 J 600 30 17197 900 10 700 60 19460 772 838 5.9 9.5 E NA 15K 600 30 17197 920 15 695 60 19360 793 874 20.6 13.0 NA NA 16 L 600 3017197 920 10 700 60 19460 786 869 7.5 11.5 NA NA 17 M 575 30 16705 90010 695 45 19239 855 925 14.6 14.2 NA NA 18 N 600 30 17197 900 10 705 6019560 820 898 5.5 10.3 NA E 19 P 575 30 16705 920 10 670 30 18576 820903 5.5 13.2 NA NA 20 A 600 30 17197 920 10 680 60 19060 863 938 9.1 7.5NA NA

Second quenching, that is, last quenching was performed on the hollowshells of respective test numbers on which the aforementioned firsttempering was performed. Specifically, the hollow shell of each testnumber was held at the quenching temperature (° C.) for the quenchingtime (min) described in the column of “last quenching” in Table 2 and,thereafter, quenching was performed on the hollow shell. All coolingspeed during quenching CR₈₀₀₋₅₀₀ in second quenching for respective testnumbers were within a range of 8 to 500° C./sec.

In addition to the above, second tempering, that is, last tempering wasperformed on the hollow shells of respective test numbers on which lastquenching was performed. Specifically, on the hollow shell of each testnumber, tempering was performed where each hollow shell was held at thetempering temperature (° C.) for the tempering time (min) described inthe column of “last tempering” in Table 2. The tempering parameters TMP₁during last tempering (=(tempering temperature (° C.)+273)×(log(tempering time (min)/60)+20)) are shown in Table 2.

Note that, in the present example, the temperature of the supplementaryheating furnace or the heat treatment furnace used for heating inquenching corresponded to “quenching temperature (° C.)”. Further, thetemperature of the heat treatment furnace used in tempering correspondedto “tempering temperature (° C.)”. Further, a time from a point of timewhen the hollow shell is charged into the holding furnace or the heattreatment furnace at the time of heating the hollow shell in a quenchingprocess to a point of time when the hollow shell is taken outcorresponded to “quenching time (min)”. A time from a point of time whenthe hollow shell is charged into the heat treatment furnace at the timeof performing tempering to a point of time when the hollow shell istaken out corresponded to “tempering time (min)”.

[Evaluation Test]

The microstructure observation, the tensile test, and the SSC resistanceevaluation test, which will be described below, were performed on theseamless steel pipes of respective test numbers on which temperingtreatment was performed.

[Microstructure Observation]

A prior γ grain diameter in the seamless steel pipe of each test numberwas measured by the aforementioned method. The prior γ grain diameters(μm) of the seamless steel pipes of respective test numbers are shown inTable 2. With respect to the seamless steel pipe of each test number,the average area of precipitates which was precipitated in prior γ grainboundaries (specific precipitates) was also acquired by theaforementioned method. The average areas of the specific precipitates(×10⁻³ μm²) in the seamless steel pipes of respective test numbers areshown in Table 2.

[Tensile Test]

Yield strength of the seamless steel pipe of each test number wasmeasured by the aforementioned method. Specifically, a tensile test wasperformed in accordance with ASTM E8/E8M (2013). More specifically, around bar tensile specimen having a parallel portion with a diameter of8.9 mm and a length of 35.6 mm was prepared from the center portion ofthe wall thickness of the seamless steel pipe of each test number. Theaxial direction of the round bar tensile specimen was parallel to theaxial direction of the seamless steel pipe.

A tensile test was performed using the round bar test specimen of eachtest number in the atmosphere at normal temperature (25° C.) to acquireyield strength (MPa) of the seamless steel pipe of each test number.Note that, in the present example, stress at 0.7% elongation acquired inthe tensile test was defined as yield strength of each test number. Theacquired yield strength YS (MPa) and tensile strength TS (MPa) are shownin Table 2.

[SSC Resistance Evaluation Test of Steel Material]

Using the seamless steel pipes of respective test numbers, a test inaccordance with “Method A” specified in NACE TM0177-2005, and afour-point bending test were performed to evaluate SSC resistance.Specifically, the test in accordance with “Method A” specified in NACETM0177-2005 was performed by the following method.

Three round bar test specimens each of which has a diameter of 6.35 mmand a parallel portion with a length of 25.4 mm were taken from thecenter portion of the wall thickness of the seamless steel pipe of eachtest number. Each round bar test specimen was taken such that the axialdirection of the round bar test specimen is parallel to the axialdirection of the seamless steel pipe. Tensile stress in the axialdirection of the round bar test specimen was applied to the round bartest specimen of each test number. At this point of operation,adjustment was performed such that stress to be applied is 90% of actualyield stress of the seamless steel pipe of each test number.

A mixed aqueous solution containing 5.0 mass % of sodium chloride and0.5 mass % of acetic acid (NACE solution A) was used as the testsolution. The test solution at 4° C. was poured into three test vessels,and these were adopted as test baths. The three round bar test specimensto which the stress was applied were immersed individually in mutuallydifferent test vessels as the test baths. After each test bath wasdegassed, H₂S gas at 1 atm was blown into the respective test baths andcaused to saturate. The test baths in which the H₂S gas at 1 atm wassaturated were held at 4° C. for 720 hours.

Meanwhile, the four-point bending test was performed by the followingmethod. Three test specimens each of which has a thickness of 2 mm, awidth of 10 mm, and a length of 75 mm, were taken from the centerportion of the seamless steel pipe of each test numbers of the wallthickness. The test specimen was taken such that the longitudinaldirection of the test specimen is parallel to the axial direction of theseamless steel pipe. Stress was applied to the test specimens of eachtest number by four-point bending in accordance with ASTM G39-99 (2011)such that stress applied to each test specimen is 90% of actual yieldstress of the seamless steel pipe of each test number. The test specimento which stress was applied was sealed into an autoclave together with atest jig.

An aqueous solution containing 5.0 mass % of sodium chloride was used asthe test solution. The test solution was poured into the autoclave whilemaintaining a gas phase portion, thus preparing test bath. After thetest bath was degassed, H₂S gas at 20 atm was pressure-sealed, and thetest bath was stirred to cause saturation of H₂S gas in the test bath.After the autoclave was sealed, the test bath was stirred for 720 hoursat 24° C.

In each of the aforementioned test in accordance with “Method A”specified in NACE TM0177-2005, and the four-point bending test, the testspecimens of respective test numbers after being held for 720 hours wereobserved with respect to presence or absence of the occurrence ofsulfide stress cracks (SSC). Specifically, the test specimens which wereheld for 720 hours were observed with the naked eyes. As a result ofobservation, a test number for which cracking was not confirmed in allof the test specimens was determined as “E” (Excellent). On the otherhand, a test number for which cracking was confirmed in at least onetest specimen was determined as “NA” (Not Acceptable).

[Test Result]

Table 2 shows the test results. With respect to the SSC resistance test,the results of the test in accordance with “Method A” specified in NACETM0177-2005 are shown in the column of “1 atm H₂S”, and the results ofthe four-point bending test are shown in the column of “20 atm H₂S”.

Referring to Table 1 and Table 2, in the seamless steel pipes of TestNumbers 1 to 8, the chemical composition was appropriate, yield strengthwas 758 to 862 MPa, the prior γ grain diameter was 11.0 μm or less, andthe average area of the specific precipitates was 10.0×10⁻³ μm² or less.As a result, excellent SSC resistance was shown in both the test inaccordance with “Method A” specified in NACE TM0177-2005 and thefour-point bending test.

On the other hand, in the seamless steel pipes of Test Numbers 9 and 10,the tempering parameter TMP₂ during second last tempering was too high.Therefore, the prior γ grain diameter was more than 11.0 μm. As aresult, excellent SSC resistance was not shown in the test in accordancewith “Method A” specified in NACE TM0177-2005.

In the seamless steel pipe of Test Number 11, the tempering parameterTMP₁ during last tempering was too high. Therefore, the average area ofthe specific precipitates was more than 10.0×10⁻³ μm². As a result,yield strength was less than 758 MPa so that yield strength of 110 ksigrade was not obtained.

In the seamless steel pipe of Test Number 12, the Cr content was toolow. Therefore, the prior γ grain diameter was more than 11.0 μm. As aresult, excellent SSC resistance was not shown in the four-point bendingtest.

In the seamless steel pipe of Test Number 13, the Cr content was toolow. Also, the Mo content was too low. In addition, F1 was too low. Inaddition, the tempering parameter TMP₁ during last tempering was toolow. Therefore, the prior γ grain diameter was more than 11.0 μm.Accordingly, the average area of the specific precipitates was also morethan 10.0×10⁻³ μm². As a result, excellent SSC resistance was not shownin either the test in accordance with “Method A” specified in NACETM0177-2005 or the four-point bending test.

In the seamless steel pipe of Test Number 14, the O content was toohigh. As a result, excellent SSC resistance was not shown in thefour-point bending test.

In the seamless steel pipe of Test Number 15, the Mo content was toolow. In addition, F1 was too low. Therefore, the prior γ grain diameterwas more than 11.0 μm. Accordingly, the average area of the specificprecipitates was also more than 10.0×10⁻³ μm². As a result, excellentSSC resistance was not shown in either the test in accordance with“Method A” specified in NACE TM0177-2005 or the four-point bending test.

In the seamless steel pipe of Test Number 16, the Cr content was toohigh. Therefore, the average area of the specific precipitates was morethan 10.0×10⁻³ μm². As a result, excellent SSC resistance was not shownin either the test in accordance with “Method A” specified in NACETM0177-2005 or the four-point bending test.

In the seamless steel pipe of Test Number 17, the Mo content was toolow. Therefore, the prior γ grain diameter was more than 11.0 μm.Accordingly, the average area of the specific precipitates was also morethan 10.0×10⁻³ μm². As a result, excellent SSC resistance was not shownin either the test in accordance with “Method A” specified in NACETM0177-2005 or the four-point bending test.

In the seamless steel pipe of Test Number 18, the Mo content was toohigh. Therefore, the average area of the specific precipitates was morethan 10.0×10⁻³ μm². As a result, excellent SSC resistance was not shownin the test in accordance with “Method A” specified in NACE TM0177-2005.

In the seamless steel pipe of Test Number 19, the V content was too low.In addition, the tempering parameter TMP₁ during last tempering was toolow. Therefore, the average area of the specific precipitates was morethan 10.0×10⁻³ μm². As a result, excellent SSC resistance was not shownin either the test in accordance with “Method A” specified in NACETM0177-2005 or the four-point bending test.

In the seamless steel pipe of Test Number 20, the tempering parameterTMP₁ during last tempering was too low. As a result, yield strength wasmore than 865 MPa so that yield strength of 110 ksi grade was notobtained. As a result, excellent SSC resistance was not shown in eitherthe test in accordance with “Method A” specified in NACE TM0177-2005 orthe four-point bending test.

An embodiment of the present invention has been described above.However, the embodiment described above is merely an example forimplementing the present invention. Accordingly, the present inventionis not limited to the above embodiment, and the above embodiment can beappropriately modified and performed within a range that does notdeviate from the gist of the present invention.

INDUSTRIAL APPLICABILITY

The steel material according to the present invention is widelyapplicable for steel materials utilized in a severe environment, such asa polar region. It is preferable that the steel material according tothe present invention can be used as a steel material utilized in an oilwell environment. It is more preferable that the steel materialaccording to the present invention can be used as a steel material, suchas a casing pipe, a tubing pipe, or a line pipe.

1.-6. (canceled)
 7. A steel material comprising: a chemical compositionconsisting of, in mass %: C: 0.20 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%,Cr: 0.60 to 1.50%, Mo: more than 1.00 to 2.00%, Ti: 0.002 to 0.020%, V:0.05 to 0.30%, Nb: 0.005 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% orless, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%,Co: 0 to 0.50%, and W: 0 to 0.50%, with the balance being Fe andimpurities, and satisfying Formula (1), wherein in the steel material, agrain diameter of a prior-austenite grain is 11.0 μm or less, an averagearea of precipitate which is precipitated in a prior-austenite grainboundary is 10.0×10⁻³ μm² or less, and a yield strength is 758 to 862MPa:2.7×C+0.4×Si+Mn+0.45×Ni+0.45×Cu+0.8×Cr+2×Mo≥3.90  (1) where, content inmass % of a corresponding element is substituted for each symbol of anelement in Formula (1), and if a corresponding element is not contained,“0” is substituted for the element symbol of the relevant element. 8.The steel material according to claim 7, wherein the chemicalcomposition contains one or more types of element selected from thegroup consisting of: Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr:0.0001 to 0.0100%, and rare earth metal: 0.0001 to 0.0100%.
 9. The steelmaterial according to claim 7, wherein the chemical composition containsone or more types of element selected from the group consisting of: Cu:0.02 to 0.50%, and Ni: 0.02 to 0.50%.
 10. The steel material accordingto claim 8, wherein the chemical composition contains one or more typesof element selected from the group consisting of: Cu: 0.02 to 0.50%, andNi: 0.02 to 0.50%.
 11. The steel material according to claim 7, whereinthe chemical composition contains one or more types of element selectedfrom the group consisting of: Co: 0.02 to 0.50%, and W: 0.02 to 0.50%.12. The steel material according to claim 8, wherein the chemicalcomposition contains one or more types of element selected from thegroup consisting of: Co: 0.02 to 0.50%, and W: 0.02 to 0.50%.
 13. Thesteel material according to claim 9, wherein the chemical compositioncontains one or more types of element selected from the group consistingof: Co: 0.02 to 0.50%, and W: 0.02 to 0.50%.
 14. The steel materialaccording to claim 10, wherein the chemical composition contains one ormore types of element selected from the group consisting of: Co: 0.02 to0.50%, and W: 0.02 to 0.50%.
 15. The steel material according to claim7, wherein the steel material is an oil-well steel pipe.
 16. The steelmaterial according to claim 7, wherein the steel material is a seamlesssteel pipe.